Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have been attracting significant attention in both academia and industry for their advantages of achieving 360{\deg} coverage and enhanced degrees of freedom. This article first identifies the fundamentals of STAR-RIS, by discussing the hardware models, channel models, and signal models. Then, three representative categorizing approaches for STAR-RIS are introduced from phase-shift, directional, and energy consumption perspectives. Furthermore, the beamforming design of STAR-RIS is investigated for both independent and coupled phase-shift cases. A general optimization framework is proposed as the recent advances, which has high compatibility and provable optimality regardless of the application scenarios. As a further advance, several promising applications are discussed to demonstrate the potential benefits of applying STAR-RIS in the sixth-generation wireless network. Lastly, a few future directions and research opportunities are highlighted for motivating future work.

翻译：同步透射与反射可重构智能表面（STAR-RIS）因其实现360{\deg}覆盖和增强自由度的优势，在学术界和工业界引起了广泛关注。本文首先通过讨论硬件模型、信道模型和信号模型，明确了STAR-RIS的基础原理。随后，从相移、方向性和能耗三个角度介绍了三种具有代表性的STAR-RIS分类方法。进一步地，针对独立和耦合相移两种情况，研究了STAR-RIS的波束赋形设计。作为最新进展，提出了一种通用优化框架，该框架具有高兼容性和可证明的最优性，不受应用场景限制。作为进一步推进，讨论了若干有前景的应用，以展示将STAR-RIS应用于第六代无线网络的潜在优势。最后，指出了几个未来方向和研究机遇，以激励后续工作。